The present invention relates to annealing of materials to improve upon material properties. More particularly, the present invention relates to a process for low-temperature, short-treatment-time annealing. Still further, the present invention relates to hydrogen annealing.
Annealing is a process that is typically used to improve the properties and characteristics of the material subject to annealing. In one type of annealing, hydrogen annealing, materials are heated at high temperature and exposed to a gas containing a hydrogen-molecule. Hydrogen annealing has been used in the manufacturing of liquid crystal displays (LCDs) to stabilize switching functions and in the manufacturing of solar cells to improve the electrical properties of solar cells. Hydrogen annealing has also been used in silicon semiconductor device manufacturing to stabilize the electrical properties of the device.
In a semiconductor manufacturing context, it is well known that exposing silicon ultra large sale integration (ULSI) devices to hydrogen gas at 400 to 450.degree. C. at the end of a wafer-processing sequence stabilizes the electrical properties of the devices. Hydrogen annealing at higher temperatures has also been studied. For example, hydrogen annealing when performed at 800 to 900.degree. C. improves the qualities of deposited and thermally grown oxides. Further, it has been reported in the following reference that hydrogen annealing at 1200.degree. C. improves the qualities of silicon wafers by removing defects near the substrate surface to the depth equivalent to the device geometry: S. Samata, M. Numano, T. Amai, Y. Matsushita, H. Kobayashi, Y. Yamamoto, T. Kawaguchi, S. Nadahara and K. Yamabe, Proc. Symp. "Degradation of Elec. Devices due to Device Operation as well as Crystalline and Process-induced Defects," ECS, Pennington, 1994, 101). The reference terms wafers subject to high temperature annealing "Hi-wafers" and ASSERTS that such wafers will be useful for ULSI device manufacturing.
The percentage of deuterium in the hydrogen annealing process is thought to have an effect upon semiconductor devices subject to such anneal. In the natural environment, the volume ratio of deuterium to hydrogen is typically about 0.015%. However, with typical hydrogen gas cylinders, the volume ratio of deuterium to hydrogen may vary widely. For example, "Rika-Nenpyo (Science Data book in Japanese)", edited by National Observatory, published by Maruzen, vol. 71, 1998) reports that typical hydrogen gas cylinders typically contain 0.0032 to 0.0184% per volume of deuterium gas. This report also suggests that by treating silicon metal-oxide-semiconductor (MOS) transistors with hydrogen gas containing a higher ratio of deuterium gas than above increases the reliability of the transistor. In particular, the report suggests that a transistor annealed in a gas having a percentage of 10 to 100 of deuterium gas, at 400 to 450.degree. C. demonstrates a ten fold increase in lifetime.
Currently, batches of wafers are annealed at the same time and not individually, for efficiency reasons. A typical hydrogen annealing process includes placing a number of semiconductor wafers into a cylindrical quartz tube, placing the tube in a furnace, heating the wafers according to a given temperature profile, maintaining the temperature at 400 to 450.degree. C. for a period of time, and cooling the wafers down according to another given temperature profile. The time may vary from dozens of minutes to several hours, depending upon the specific recipe. Because a significant portion of the anneal time is used for heating up the wafers to a particular temperature and cooling the wafers down, annealing is a very time consuming process. As a result, single-wafer by single-wafer processing that is widely used in etching processes and chemical vapor deposition (CVD) processes, has not been suitable for the conventional hydrogen annealing.
One drawback with conventional hydrogen annealing processes is the high cost of the hydrogen gas used. Even when a diluted hydrogen gas is used, because the treatment or anneal time is long, a large volume of gas is still used. When deuterium is used for annealing, which is much more expensive than hydrogen, the cost of the gas used is even greater. For example, the cost of deuterium gas for annealing a hundred 8-inch wafers for an hour is estimated to be $20,000 to $30,000.
Another drawback with conventional hydrogen annealing processes is that equipment costs are high. For example, with conventional hydrogen annealing, the flow rate of these gases (hydrogen or deuterium) needed for typical multiple-wafer processes must be large. Because of this, the exhaust system used for such gases must have a large capacity. The installation of such large exhaust systems raises the capital and operational costs of the fabrication facility.
Another drawback of the conventional hydrogen annealing is that the high process temperature may cause other defects in the device. As disclosed above, typical hydrogen annealing process uses temperatures ranging from 400 to 450.degree. C. These temperatures are typically below the melting point of aluminum that is used for interconnection in ULSI devices, however these temperatures are sufficient to cause deformations of aluminum grain boundaries on the wafer. As a result of these deformations, hillocks may appear in the aluminum grains. When such hillocks appear in grain boundaries used for ULSI interconnects, these hillocks may cause stress migration of aluminum, may break the interconnection due to thermal expansion coefficients mismatch between aluminum and underlying materials, and the like.
Yet another drawback to conventional hydrogen annealing is that handling of the multiple wafers can introduce extra defects. As disclosed above, single-wafer by single wafer processing that is widely used in etching processes, chemical vapor deposition (CVD) processes, and the like. These processes typically utilize multiple chamber machines that can isolate the single wafers from the atmosphere between processes, and cluster-type tools. Because of the increased isolation of the wafers during such processes, it has been demonstrated that higher wafer reliability is achieved, lower native oxide growth appears, and the like.
One modification proposed to address some of these drawbacks has been to perform hydrogen annealing at a lower temperature, for example 200.degree. C. A drawback to this proposal is that it appears that the annealing is ineffective. In particular, it does not appear that enough hydrogen is diffused into a device to cause improvement in the electrical properties of silicon dioxide and the interface between silicon and silicon dioxide.
Another modification proposed to address some of these drawbacks has been to perform hydrogen annealing at a higher temperature, for example 1200.degree. C. A drawback to this "Hi-wafer" process is that special higher cost processing equipment is required. For example, instead of using quartz tubes that are conventionally used within the annealing furnace, special high temperature tubes must be used. As an example, high temperature materials include silicon carbide (SiC), which is quite expensive compared to quartz.
Thus what is required in the industry are more efficient, economical, and effective ways to anneal semiconductor wafers using a hydrogen gas or a deuterium gas.